Impact of peak respiratory exchange ratio on the prognostic power of symptoms-limited exercise testing using Bruce protocol in patients with Fontan physiology

OBJECTIVES: We evaluated the impact of peak respiratory exchange ratio on the prognostic values of cardiopulmonary exercise variables during symptoms-limited incremental exercise tests in patients with Fontan physiology. METHODS: Retrospective single-centre chart review study of Fontan patients who underwent exercise testing using the Bruce protocol between 2014 and 2018 and follow-up. RESULTS: A total of 34 patients (age > 18 years) had a Borg score of ≥7 on the Borg 10-point scale, but only 50% of patients achieved a peak respiratory exchange ratio of ≥ 1.10 (maximal test). Peak oxygen consumption, percent-predicted peak oxygen consumption, and peak oxygen consumption at the ventilatory threshold was reduced significantly in patients with a peak respiratory exchange ratio of < 1.10. Peak oxygen consumption and percent-predicted peak oxygen consumption was positively correlated with peak respiratory exchange ratio values (r = 0.356, p = 0.039). After a median follow-up of 21 months, cardiac-related events occurred in 16 (47%) patients, with no proportional differences in patients due to their respiratory exchange ratio (odds ratio, 0.62; 95% CI: 0.18-2.58; p = 0.492). Multivariate Cox proportional hazard analysis showed percent-predicted peak oxygen consumption, peak heart rate, and the oxygen uptake efficient slope were highly related to the occurrence of events in patients only with a peak respiratory exchange ratio of ≥ 1.10. CONCLUSIONS: The value of peak cardiopulmonary exercise variables is limited for the determination of prognosis and assessment of interventions in Fontan patients with sub-maximal effort. Our findings deserve further research and clinical application.